1. Field of the Invention
This invention relates generally to concurrent access to shared objects, and more particularly to a system and method for implementing a transactional memory that includes mechanisms to support dynamic mixed mode read validation.
2. Description of the Related Art
The multi-core revolution currently in progress is making it increasingly important for applications to exploit concurrent execution in order to take advantage of advances in technology. Shared-memory systems allow multiple threads to access and operate on the same memory locations. To maintain consistency, threads must often execute a series of instructions as one atomic block, or critical section. In these cases, care must be taken to ensure that other threads do not observe memory values from a partial execution of such a block. Such assurances are important for practical and productive software development because without them, it can be extremely difficult to manage the interactions of concurrent threads. Traditional constructs, such as mutual exclusion and locks may be used by a thread to ensure correctness by excluding all other threads from concurrent access to a critical section. For example, no thread may enter a critical section without holding the section's lock. While it does, all other threads wishing to execute the critical section must await the lock's release and acquire it before proceeding.
The pitfalls of these constructs are numerous and well known. They include dead-lock, priority inversions, software complexity, and performance limitations. Locking large sections of code and/or code that accesses a lot of data is a heavy-handed approach to concurrency control.
Alternatively, it may be possible to increase parallelism by allowing multiple threads to execute a critical section at one time if the executions do not rely on overlapping memory locations. This may increase performance and mitigate many of the pitfalls normally associated with traditional locking mechanisms. However, such interleaved executions are not guaranteed to be correct.
Transactional memory is a mechanism that can be leveraged to enable concurrent and correct execution of a critical section by multiple threads. Transactional memory allows a thread to execute a block of instructions either completely and atomically or not at all. As typically defined, a transactional memory interface allows a programmer to designate certain sequences of operations as “atomic blocks” and “transactions,” which are guaranteed by the transactional memory implementation to either take effect atomically and in their entirety (in which case they are said to succeed), or to be aborted, such that they have no externally visible effect (in which case they are said to fail). Thus, with transactional memory, it may be possible in many cases to complete multiple operations with no possibility of another thread observing partial results, even without holding any locks. The transactional memory paradigm can significantly simplify the design of concurrent programs. In general, transactional memory can be implemented in hardware (HTM), in software (STM), or in any of a variety of hardware-assisted software implementations.